1. Lets giix do the job for the form/controller for the order and the categories related to the order (checkboxes).
Comment the tbl_order_categories before creating the CRUD with the help of giix, so that the code for OrderProduct in the form/controller will not be created.

line 744: renderHiddenPk - will work for composit PK
line 286: $item->save() - should work too with composit PK
line 291: needs testing
if (!empty($deleteItems))
foreach($deleteItems as $pk)

Sorry, I have no time in the moment to build a test enviroment for this, but your code seems to be one ;-)
So, if you do the modification(s) and send me your working/tested new code of the multimodelform, I will add this to the next release.

Thank you for plugin! Wish you having time and strength to continue supporting this plugin!

Also I have one question: is it possible to clone several model input fields, but not all of them?
For example, if I have 10 input fields, and 7 of them are "static" (text will be entered only once) and another 3 will be copied by pressing "add item".

I didn't test it, but should be possible.
...
There exists the property 'excludeSelector' and you can set the jqrelcopy-properties through the 'options' property of the multimodelform widget.
But maybe this will have sideeffects on building the items-arrays on validate/save... and needs more workaround.

One question, how I can do to change the form view related record, I have many fields and I want them for example in groups of three or more in vertical?, something like what is shown in the picture. is it possible?